A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench ⁵⁷Fe Mössbauer Data, and a Hydride Resting State
Abstract
The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe–N₂ complexes of tetradentate P₃ᴱ ligands (E = B, C) generate catalytic yields of NH₃ under an atmosphere of N₂ with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of magnitude improvement in yield of NH₃ for each Fe catalyst has been realized (up to 64 equiv of NH₃ produced per Fe for P₃ᴮ and up to 47 equiv for P₃ᶜ) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P₃ᴮFe–N₂/P₃ᴮFe–N₂– couple and controlled-potential electrolysis of P₃ᴮFe⁺ at −45 °C demonstrates that electrolytic N₂ reduction to NH₃ is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P₃ᴮ), consistent with a single-site catalyst model. An isostructural system (PP₃^(Si)) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-quench Mössbauer spectroscopy during turnover reveals an iron–borohydrido–hydride complex as a likely resting state of the P₃ᴮFe catalyst system. We postulate that hydrogen-evolving reaction activity may prevent iron hydride formation from poisoning the P₃ᴮFe system. This idea may be important to consider in the design of synthetic nitrogenases and may also have broader significance given that intermediate metal hydrides and hydrogen evolution may play a key role in biological nitrogen fixation.
Additional Information
© 2016 American Chemical Society. Received: February 16, 2016; Publication Date (Web): March 29, 2016. This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. T.J.D.C. acknowledges the support of the NSF for a Graduate Fellowship (GRFP), and N.B.T. acknowledges the support of the Resnick Sustainability Institute at Caltech for a Graduate Fellowship. T.J.D.C. and N.B.T. contributed equally to this work. The authors declare no competing financial interest.Attached Files
Accepted Version - nihms824014.pdf
Supplemental Material - ja6b01706_si_001.pdf
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Additional details
- Alternative title
- A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench 57Fe Mössbauer Data, and a Hydride Resting State
- PMCID
- PMC5079282
- Eprint ID
- 66312
- Resolver ID
- CaltechAUTHORS:20160420-105550849
- NIH
- GM 070757
- Gordon and Betty Moore Foundation
- NSF Graduate Research Fellowship
- Resnick Sustainability Institute
- Created
-
2016-04-20Created from EPrint's datestamp field
- Updated
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2022-04-28Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute